raise error if --http2 is specified, but the OpenSSL version doesn't support it

libmproxy/cmdline.py

@@ -363,6 +363,11 @@ def proxy_options(parser):
         help="Proxy service port."
     )
     http2 = group.add_mutually_exclusive_group()
+    # !!!
+    # Watch out: We raise a RuntimeError in libmproxy.proxy.config if http2 is enabled,
+    # but the OpenSSL version does not have ALPN support (which is the default on Ubuntu 14.04).
+    # Do not simply set --http2 as enabled by default.
+    # !!!
     http2.add_argument("--http2", action="store_true", dest="http2")
     http2.add_argument("--no-http2", action="store_false", dest="http2",
                        help="Explicitly enable/disable experimental HTTP2 support. "

libmproxy/proxy/config.py

@@ -180,6 +180,9 @@ def process_proxy_options(parser, options):
             parser.error("Certificate file does not exist: %s" % parts[1])
         certs.append(parts)
 
+    if options.http2 and not tcp.HAS_ALPN:
+        raise RuntimeError("HTTP2 support requires OpenSSL 1.0.2 or above.")
+
     return ProxyConfig(
         host=options.addr,
         port=options.port,